Process for preparing titanium dioxide

ABSTRACT

TITANIUM TETRACHLORIDE IS OXIDISED TO TITANIUM DIOXIDE IN A BURNER-HEATED CHAMBER OF PARTICULAR DESIGN PREVENTING FORMATION OF DEPOSITS ON THE WALLS AND IN THE FEED PASSAGES FOR TITANIUM TETRACHLORIDE.

- May 22, 1973 B. CALCAGNO T 3, 5, 00

PROCESS FOR PREPARING TITANIUM DIOXIDE Filed July 14. 1970 United States Patent US. Cl. 423-613 6 Claims ABSTRACT OF THE DISCLOSURE Titanium tetrachloride is oxidised to titanium dioxide in a burner-heated chamber of particular design preventing formation of deposits on the walls and in the feed passages for titanium tetrachloride.

This invention relates to a process for the preparation of titanium dioxide by the oxidation of titanium tetrachloride in gaseous phase, by means of oxygen, at high temperature.

Various methods are known whereby metallic oxides can be produced from the appropriate halides, by means of oxidising gases, at temperatures ranging from 100 C. to 1,400 C.

In these processes, the high temperatures needed to bring the reaction about are obtained either by preheating at least one of the reagents or by the use of a flame from an auxiliary combustible.

The present invention is concerned with those processes in which the reaction takes place in the presence of a flame from an auxiliary combustible, in which the titanium tetrachloride is oxidised to titanium dioxide.

Industry has many uses for titanium dioxide, especially as a pigment, for example in papers, varnishes, gums and plastics.

For use as a pigment, a product having special characteristics is required, such as, for instance, those connected with covering power, dispersion, brilliance and so forth.

One object of the present invention is therefore a method of oxidising titanium tetrachloride so as to produce titanium dioxide of suitable crystalline structure, such as rutile, in the form of fine particles of regular, rounded shape, of definite diameter within a limited range of values, and free from aggregates.

Another object of the invention is a method of oxidising titanium tetrachloride whereby the formation of deposits is avoided in the equipment used, thus making it possible to obtain titanium dioxide having characteristics which remain constant with time.

A further object of the invention is a. form of equip-- ment, simple in design and easy to maintain, suitable for the oxidation of titanium tetrachloride, for the production of titanium dioxide having the characteristics already mentioned.

Further objects of the invention will become apparent from the description which follows.

The process here proposed consists essentially in bringing into contact, within suitable equipment, a relatively cold stream of gas consisting of titanium tetrachloride and the oxidising gas and a stream of hot gases obtained by burning the auxiliary combustible, this contact being brought about by a special system of flow dynamics.

More specifically, the process here proposed consists in forming within a single chamber (reaction chamber) three distinct zones consisting of: an initial zone, in which what occurs is substantially the burning of the auxiliary combustible (combustion zone); a second zone, in which what occurs is substantially the mixing of the stream of gas consisting of the titanium tetrachloride and the oxidising gas with the stream of hot gases derived from the burning of the auxiliary combustible (mixing zone); and

the third zone, in which what takes place is substantially the oxidation of the tetrachloride to titanium dioxide (oxidation zone).

In more precise terms, the reaction chamber comprises an upper zone, cylindrical in shape (combustion zone), communicating with a lower zone, likewise cylindrical, but larger in diameter (oxidation zone), these two zones being joined together by a truncated cone, which constitutes the mixing zone.

For a clearer understanding of the process, the invention will now be described in conjunction with the accompanying drawings.

FIG. 1 is a sectional diagram of the reaction chamber, consisting of the combustion zone 1, the mixing zone 2 and the oxidation zone 3.

More particularly, the top part of the combustion zone 1 contains the burner 4 for the auxiliary combustible or fuel. The combustion zone 1 is cylindrical, its height being such that the flame is substantially spent before contact is made with the titanium tetrachloride.

It is preferable to use burners of pre-mix type, since they enable fairly short flames to be obtained.

Possible fuels are: carbon monoxide, hydrogen, methane, acetylene or liquefied petroleum gases. Carbon monoxide is generally preferable, because it does not lead to the formation of hydrochloric acid, which is undesira-ble as a by-product.

A basic feature of the process with which the invention is concerned is that the titanium tetrachloride is fed in the form of an annular laminar flow of gas inclined at 30 to 60 to the central stream of hot gases produced by the burning of the fuel.

Laminar flow of gas here means flows having a thickness of from 0.1 cm. to 0.8 cm.

With reference to FIG. 1, the titanium tetrachloride fed in through 6 is conveyed into the mixing zone 2 through the annular slit 7 contained in the wall of this frusto-conical zone, the said slit running Substantially at right angles to that wall.

More particularly, the Slope of that wall in relation to the longitudinal center line of the reaction chamber is so determined that the desired angle of impact is produced between the stream of titanium tetrachloride and the stream of hot reaction gases.

Another basic feature of the invention is that the oxygen intended for oxidising the titanium tetrachloride is fed in tangentially to the wall of the upper part of the combustion zone.

With reference to FIG. 1, the cold oxygen is fed in through two pipe connections, 5, details of which are brought out more clearly in FIG. 2, which is a section through the reaction chamber at the level of the oxidising gas feed.

The flow of oxidising gas follows a path close to the wall until it reaches the zone in which it is mixed with the titanium tetrachloride.

In that way, damage to the combustion zone wall by the flame from the fuel is avoided and the oxygen, at a temperature substantially below that at which the oxidising reaction takes place, is guided by the flow of titanium tetrachloride gas, entering through 7 into the mixing chamber 2, into contact with the stream of hot gases passing down the center of the reaction chamber.

This arrangement also protects the feed slit for the titanium tetrachloride from the formation of deposits.

To achieve these purposes, it is necessary to maintain the linear entry velocity of the gaseous flow of titanium tetrachloride into the reaction chamber at a value of not less than metres a second, though it is undesirable, for satisfactory reaction, to exceed 70 metres a second.

In this way, it is possible to achieve an initial rapid mixing of the oxygen with the titanium tetrachloride at a temperature below that at which titanium dioxide is formed, followed by a second rapid mixing of these reagents with the hot gases produced by combustion of the fuel, so that oxidation temperature is achieved.

In the application of the present invention, the molar ratio observed between the oxygen and the titanium tetrachloride is preferably between 1.15 :1 and 1.421, the oxygen being fed in at or slightly above ambient temperature, at C. to 40 C., for example, while the titanium tetrachloride may suitably be heated to temperatures in no case exceeding 500 C.

Oxygen or gases containing more than 90% of molecular oxygen are employed for the oxidation reaction.

It is also essential, for the purposes of the present invention, to regulate the fuel combustion so as to obtain rapid pre-heating of the reagents in the mixing zone to the temperature at which oxidation of the titanium tetrachloride takes place.

In the particular case in which carbon monoxide is used as the auxiliary fuel and the titanium tetrachloride is preheated to 300 C.-400 C., it has been found that the molar ratio between the carbon monoxide fed in for combustion and the titanium tetrachloride fed in for oxidation should never be lower than 0.3:1, if reaction products having the required characteristics are to be obtained.

As regards the oxidation temperature, this should be between 1,200 C. and 1,400 C.

In the tetrachloride oxidation reaction in the prior art, the formation of titanium dioxide is known to occur within a temperature range above its flow point, at which it has plasticity, so that it tends to cling to any surfaces with which it comes in contact.

Deposits may therefore form on the burner and on the walls of the reaction chamber, especially at points where the reagent substances also make contact.

Deposits forming on the burner are extremely damaging, since they intefere with the flame formation, causing instability and irregularity.

The result is a lack of consistency in the characteristics of the end product.

The effects are similar when deposits form in the vicinity of the slits through which the reagents are admitted.

In the present instance, by reason of the geometrical arrangement, the formation of the titanium dioxide takes place far enough from the burner to safeguard it from the deposition of solids.

Deposition is also avoided in the feed passages of the titanium tetrachloride, since, as previously stated, the reagents come into contact in that zone at temperatures substantially below the reaction temperature, apart from the protective action of the entry velocity.

Finally, as the reaction takes place within a zone restricted to the central part of the chamber, the formation of deposits on the walls of the oxidation zone is prevented.

To obtain titanium dioxide having the characteristics described earlier, apart from flame control and the addition, in accordance with standard practice, of suitable nucleus-forming agents as, for example, the chlorides of aluminium or silicon or water vapour, it is necessary to establish a system of flow dynamics such as to alford equal conditions for all the particles as regards formation, growth, high-temperature stability and cooling.

For this cooling, the same gases as are produced in course of reaction may be used, these being re-cycled at low temperature after dust-extraction.

Finally, the titanium dioxide is separated from the other reaction products and subjected to quality tests for use in pigments, after which it is treated by the usual methods to obtain the finished pigment.

4 The following examples of experiments will serve to illustrate the invention further, but they should not be regarded as implying any limitation on the scope thereof.

EXAMPLE 1 The reaction chamber used for this experiment was of the type shown in the accompanying drawings.

In particular, referring to FIG. 1, the combustion zone was 15 cm. in diameter and 25 cm. long, the oxidation zone was 35 cm. in diameter and 120 cm. long and these were joined together by a 45 truncated cone.

The chamber was made from refractory material containing 70% to 72% of alumina.

The annular feed slit for the titanium tetrachloride was 0.1 cm. wide and was bounded by two walls of nickel, trued and suitably spaced so as to provide a constant aperture round the entire circumference.

The burner was fed with carbon monoxide, containing 2% of hydrogen, and oxygen in quantities gradually rising to maximum values of 22.6 N cu.m./hour for the carbon monoxide and 17 N cu.m./hour for the oxygen.

The burner was of the pre-mix type and provided a flame which was substantially spent within the combustion zone, so that the gas entered the mixing zone fully combusted.

At the same time, oxygen was fed in tangentially, at the rate of 47 N cu.m./hour, through two pipe connections. fitted at the top of the combustion zone.

The oxygen feed path can be seen particularly clearly in FIG. 2.

At this stage, while getting underway, a stream of nitrogen was introduced through the annular slit in the wall of the frusto-conical zone.

When, after about two hours, steady-state conditions had been achieved, the nitrogen was gradually replaced by titanium tetrachloride.

The titanium tetrachloride, containing 2% by weight of aluminium trichloride, was pre-heated to a temperature of 400 C. in Inconel heat exchangers and fed in at the rate of 320 kg./hour.

These heat exchangers are not shown in the accompanying drawings.

In more precise terms, the metal halide was fed through an annular slit 0.1 cm. wide, set at right angles to the wall of the frusto-conical zone, the stream of gas emerging at a linear velocity of 33 m./ sec.

The stream of titanium tetrachloride was admitted to the mixing zone in a direction inclined at 45 to the axis of the stream of hot gases produced by the combustion of the carbon monoxide.

In this experiment, a molar ratio of 0.621 was maintained between the carbon monoxide and titanium tetrachloride.

The reaction products were cooled to about 300 C. in the delivery pipes from the chamber by re-cycling the reaction gases after cooling and dust extraction.

In this way, 134 'kg/hour of titanium dioxide was obtained, the characteristics of which were as follows:

mean particle size: 0.23 micron;

particle size distribution: of the particles were of sizes between 0.17 and 0.30 micron, without any aggregates Whatever;

crystalline form: rutile as to more than 99%;

Reynolds number: 1950.

The product gave excellent results in pigment applications.

When the run was stopped, after 10 hours, no deposits were observed in the equipment.

EXAMPLE 2 The same procedure was followed and the same equipment used as in Example 1, but less fuel was fed to the burner.

The molar ratio maintained between the carbon monoxide and the titanium tetrachloride was 0.3 1.

The titanium dioxide obtained had characteristics equal to those in Example 1..

EXAMPLE 3 The same procedure was followed and the same equipment used as in Example 1, but less fuel was fed to the burner.

The molar ratio maintained between the carbon monoxide and the titanium tetrachloride was 03:1.

The titanium dioxide obtained had a Reynolds dye number of 1800 and 80% of the particles had a size distribution of 0.15 to 0.30 micron.

Some aggregates were observed.

EXAMPLE 4 The same procedure was followed and the same equipment used as in Example 1, but the supply of carbon monoxide was reduced.

The molar ratio maintained between the carbon monoxide and the titanium tetrachloride was 0.2: 1.

The titanium dioxide obtained had a Reynolds dye number of 1300 and 80% of the particles had a size distribution of 0.10 to 0.32 micron.

Moreover, there were numerous aggregates and some of the particles were irregular in shape.

What We claim is:

1. In a process for producing titanium dioxide comprising oxidizing gaseous titanium tetrachloride With gaseous oxygen at a high temperature by means of and in the presence of a stream of hot gases produced in a reactor by the combustion of an auxiliary combustible, the improvement comprising avoiding the formation of titanium dioxide deposits in the reactor and obtaining titanium dioxide having characteristics which remain constant with time by a process which comprises:

(1) dividing the reactor into an upper combustion zone comprising a cylindrical zone of a first diameter, a lower oxidation zone comprising a cylindrical zone of a second diameter larger than said first diameter, and an intermediate mixing zone comprising a frustoconical zone increasing in diameter from said first diameter to said second diameter;

(2) igniting and completely combusting said auxiliary combustible in said combustion zone forming a stream of hot gas flowing downwardly through the central portion of said combustion zone into the central portion of said mixing zone, said combustion zone being of a sufficient length such that the flame of said igniting does not extend into said mixing zone;

(3) feeding gaseous oxygen, at or slightly above ambient temperature, into the upper portion of said combustion zone tangentially t the interior wall thereof forming a stream of oxygen which flows downwardly through said combustion zone without mixing with said stream of hot gas, said stream of oxygen flowing adjacent the interior wall of said combustion zone into and adjacent the interior wall of said mixing zone;

(4) feeding gaseous titanium tetrachloride pre-heated to a temperature not exceeding 500 C. into said mixing zone at a linear velocity of from 10 to meters per second through a continuous annular slit formed in the sloping wall of said mixing zone, said slit being substantially at right angles to said sloping wall, whereby said oxygen and said titanium tetrachloride mix Without substantial reaction forming an annular flow of gas 0.1 to 0.8 cm. in thickness flowing from the wall of said mixing zone toward the center thereof at which point said gas mixture mixes with the hot gas stream flowing downwardly through the central portion of said mixing zone, the angle formed between said hot gas stream and said annular stream at mixing being from 30 to 60, said mixing zone being of sufiicient length to provide substantially complete mixing of said annular stream and said hot gas stream within the central portion of said mixing zone;

(5) oxidizing said titanium tetrachloride in said oxidation zone at a temperature of from 1200 to 1400 C.; and

(6) recovering titanium dioxide from the cooled reaction products.

2. A process as claimed in claim 1 in-which the auxiliary combustible employed is selected from the group consisting of carbon monoxide, hydrogen, methane, acetylene and liquefied petroleum gases.

3. A process as claimed in claim 1 wherein the oxygen is fed into the upper portion of said combustion zone at a temperature of from 15 to 40 C.

4. A process as claimed in claim 1 wherein said auxiliary combustible is carbon monoxide, wherein the titanium tetrachloride is pre-heated to a temperature of from 300 to 400 C. and wherein the molar ratio between said car bon monoxide and said titanium tetrachloride is at least 0.3 /1.

5. A process as claimed in claim 1, in which the molar ratio between the oxygen and the titanium tetrachloride lies between 1.15:1 and 1.4:1.

6. A process as claimed in claim 1, in which oxygen or a gas containing more than of molecular oxygen is used for the oxidation.

References Cited UNITED STATES PATENTS 3,481,703 12/1969 Zirngibl et al. 23202 3,485,584 12/ 1969 Zirngibl et al. 23202 3,512,219 5/1970 Stern et a1 23-202 V X 3,582,278 6/1971 Kulling et al 23202 V FOREIGN PATENTS 1,181,771 2/ 1970 Great Britain 23202 EDWARD STERN, Primary Examiner US. Cl. X.R. 106-300; 23277 

